A sail is essentially a flexible surface capable of assuming, when exposed to a fluid flow, and more particularly to wind flow, a form capable of generating a propulsive force; generally, the sail assumes in the wind a wing-shaped form that generates lift.
For this reason, when designing, producing and using a sail (or indeed a set of sails interacting with each other) it is extremely important to anticipate and analyse the behaviour of the effective form of the sail once exposed to the wind flow in order to optimise its structure, its profiles, its adjustments and, more generally, its form.
In Anglo-Saxon terminology, the terms “flying shape” and “design shape” are used to distinguish the real form of the sail when hoisted and exposed to the wind from the geometric form conceived by the sailmaker and/or designer. It is evident that the flying shape may diverge more or less markedly from the design shape due to the inevitable approximations of the design, the adjustments to which the sail is subjected, the materials used and the real wind conditions.
Detection of the real form of the sail is a technique adopted in relatively recent times in order to attempt to improve the design of the sail and the adjustments to which it is subjected, with the aim of maximising its efficiency.
To this end, various methods and devices have been developed, generally based on imaging systems using photographic or video cameras that collect images of the sail for subsequent processing of their content. One of the more successful systems is known as VSPARS, developed with the support of the Yacht Research Unit of the University of Auckland in New Zealand. This is based on photographic exposures made from the foot of the sail in action; the sail is provided with “shape lines” consisting of coloured stripes contrasting with the background of the sail, applied to its surface in significant areas, generally spaced at intervals along the vertical profile of the sail. This technique has proved effective, but is subject to certain limits; in particular due to the fact that the entire form of the sail is not processed, but only that of the shape lines. The rest of the information relating to the entire surface of the sail is obtained by processing the recorded data.